The present invention relates to travelling wire EDM apparatus, or apparatus for machining, by electrical discharges, an electrode workpiece by means of an electrode wire. More particularly, the present invention is addressed to a mechanism for automatically changing the electrode wire of a travelling wire EDM apparatus without human intervention.
It is known, in travelling wire electrical discharge technology, that it is advantageous to select an electrode wire best suited to the work to be effected. For example, while making a die for use in a stamping press, minimum radii of the co-operating punch and die are determined as a function of the thickness of the sheet steel stock to be cut. The greater the thickness of the sheet of steel, the larger are the minimum radii. Consequently, the punch and the die, which are obtained by electrical discharge machining, must have the dimensional and shape details which are appropriate for the part to be stamped, and the diameter of the electrode wire used for cutting the die must preferably conform to the desired degree of definition of the fine details of the die. Extremely detailed dies require the use of very slender electrode wires.
It is even sometimes necessary to use electrode wires of different compositions. For example, if it is desired to obtain very fine details, it is advantageous to use a molybdenum electrode wire, rather than a more conventional bronze or copper electrode wire, as molybdenum is a metal having very high strength under traction which thus permits to adopt an electrode wire of very small diameter.
Japanese Pat. No. 69.037-81 discloses a travelling wire EDM apparatus with automatic threading of the electrode wire and with the capability of selecting an electrode wire from several available wires. However, the disclosed structure is cumbersome and expensive because it results in a redundant multiplication of the number of wire feeding mechanisms for feeding the electrode wires to the machining zone of the apparatus. In other words, for each electrode wire capable of being used, there is a separate supply, feeding and guiding mechanism, such that the number of wire feeding mechanisms is a direct function of the number of separate wires which are capable of being used by the apparatus.